It has become apparent that in future, both in the case of stationary applications and also in the case of vehicles such as hybrid vehicles and electric vehicles, battery systems will be used ever more frequently. In order to be able to meet particular requirements for a respective application voltage and the power that can be made available, a high number of battery cells are connected in series. Since it is necessary for the current that is provided by a battery of this type to flow through all the battery cells and a battery cell can only carry a limited amount of current, battery cells are in addition often connected in parallel in order to increase the maximum current. This can be achieved either by providing a plurality of battery cells within a battery cell housing or by connecting battery cells externally.
FIG. 1 illustrates the principal circuit diagram of a conventional electric drive system, such as is used for example in electric vehicles and hybrid vehicles or also in stationary applications such as when adjusting rotor blades of wind turbines. A battery 110 is connected to a DC voltage intermediate circuit and said DC voltage intermediate circuit is embodied by a capacitor 111. A pulse-controlled inverter 112 is connected to the DC voltage intermediate circuit and sinusoidal voltages that are phase-offset with respect to each other for operating an electric drive motor 113 are supplied by said pulse-controlled inverter 112 to three outputs by way of in each case two switchable semi-conductor gates and two diodes. The capacity of the capacitor 111 that forms the DC voltage intermediate circuit must be sufficiently large in order to stabilize the voltage in the DC voltage intermediate circuit for a period of time in which one of the switchable semi-conductor gates is switched to conduct. In a practical application, such as an electric vehicle, a high capacity in the mF range is achieved.
FIG. 2 illustrates the battery 110 of FIG. 1 in a detailed block diagram. A plurality of battery cells is connected in series and optionally in addition in parallel in order to achieve a battery capacity and a high output voltage required for a respective application. A charging and disconnecting device 116 is connected between the positive pole of the battery cells and a positive battery terminal 114. Optionally, a disconnecting device 117 can in addition be connected between the negative pole of the battery cells and a negative battery terminal 115. The disconnecting and charging device 116 and the disconnecting device 117 comprise in each case a switch 118 or 119 respectively, which switches are provided for disconnecting the battery cells from the battery terminals in order to disconnect the battery terminals from the voltage supply. Otherwise, as a result of the high DC voltage of the battery cells that are connected in series, there is a considerable potential risk for maintenance personnel or the like. A charging switch 120 having a charging resistor 121 that is connected in series to the charging switch 120 is in addition provided in the charging and disconnecting device 116. The charging resistor 121 limits a charging current for the capacitor 111 if the battery is connected to the DC voltage intermediate circuit. For this purpose, the switch 118 is initially left open and only the charging switch 120 is closed. If the voltage at the positive battery terminal 114 achieves the voltage of the battery cells, the switch 119 can be closed and if necessary the charging switch 120 can be opened.
The charging switch 120 and the charging resistor 121 represent a significant amount of additional expenditure in applications in which the output is in the range of a few 10 kW and said additional expenditure is only required for the process that lasts a few hundred milliseconds for charging the DC voltage intermediate circuit. Said components are not only expensive but they are also large and heavy which is particularly troublesome when used in mobile applications such as electric motor vehicles.